This invention relates to the field of polymer blends or alloys containing ethylene-vinyl acetate (EVA) copolymer and/or ethylene-vinyl acetate-vinyl alcohol (E-VA-VOH) terpolymer as impact strength modifier for brittle styrene-acrylonitrile copolymer.
It is well known that relatively few polymerpolymer blends have useful properties due to their incompatibility. Even useful polymer blends, such as those of nitrile rubber in styrene-acrylonitrile (SAN) copolymer to increase impact strength, have been greatly improved by substituting rubber-SAN graft copolymers for the nitrile rubber to provide superior impact resistance. These well known compositions are now generally referred to as the ABS (acrylonitrile-butadiene-styrene) resins.
Techniques other than that of graft polymerization have been employed for introducing compatibility in polymer blends, or alloys. Thus, for example, as described in U.S. Pat. No. 3,906,057, a block copolymer which combines blocks of two homopolymers in a single macromolecule will compatibilize a blend of the homopolymers when added in amounts of about 5-40 weight parts per hundred parts of blend. Similarly, U.S. Pat. No. 3,485,777 describes the compatibilization of polymer blends by the addition of a graft copolymer having molecular segments of solubility characteristics similar to the solubility characteristics of the blended component polymers. Variations of these compatibilization techniques include coupling polymer components in situ with coupling agents as described in U.S. Pat. No. 3,645,939
These methods of obtaining compatibilization in polymer blends are of limited usefulness since they each depend upon some special additive and extensive working or a chemical reaction to achieve the result desired. Accordingly, attempts have been made in the past to obtain compatibilized polymer blends by merely blending the component polymers using simple known and conventional procedures. A number of polymer alloys containing EVA copolymer or E-VA-VOH terpolymer and one or more other polymers are known. U.S. Pat. No. 3,283,035 describes a polymer blend of polyvinyl chloride (PVC) containing at least 80 weight percent vinyl chloride and optionally having incorporated therein minor quantities of a copolymerizable monomer such as acrylonitrile, together with 2 to 40 parts by weight of said PVC of an EVA copolymer containing 2 to 65 weight percent vinyl acetate and 3 to 10 parts by weight of said PVC of a chlorinated polyethylene (CPE) containing 35 weight percent chlorine. No mention is made of the use of a SAN copolymer in the aforesaid polymer blends. U.S. Pat. No. 3,322,858 describes multicomponent polymer blends containing vinyl chloride grafted EVA copolymers of up to 50 weight percent EVA copolymer (with the working examples indicating a preference for substantially smaller quantities of EVA copolymer in the graft resin), a CPE resin and a SAN copolymer as a processing aid. U.S. Pat. No. 3,549,727 discloses polymer blends containing E-VA-VOH terpolymer. The use of PVC, CPE and/or SAN copolymer are neither disclosed nor suggested. U.S. Pat. No. 3,644,577 describes polymer blends containing 60 to 95 weight percent PVC and 40 to 5 weight percent of a nitrile copolymer such as SAN copolymer. The blends may also contain as an optional ingredient an impact modifier such as an EVA copolymer. The characteristics of the polymers disclosed in this patent are largely those of PVC reflecting the dominant presence of this resin in the blends. U.S. Pat. No. 3,758,661 is illustrative of a two component blend containing a copolymer of an alpha-olefin and a copolymerizable monomer therewith, e.g., EVA copolymer, and a resin having poor compatibility with the olefinic resin such as SAN copolymer of PVC. U.S. Pat. No. 3,960,986 describes impact-resistant molding compositions containing 20 to 80 percent PVC, 19.5 to 75 weight percent of a vinyl chloride graft copolymer of ethylene and vinyl acetate and 0.5 to 5.0 weight percent EVA copolymer. The incorporation of nitrile-containing copolymer such as SAN copolymer is not mentioned in this patent.
In accordance with U.S. Pat. No. 4,141,936, compatibilized blends of certain EVA copolymers and/or E-VA-VOH terpolymers in SAN copolymer are obtained by blending PVC or CPE as the third component of the blend. The polymer triblends of this patent possess enhanced impact strength and injection moldability without any loss of heat aging resistance. Some compositions of the compatibilized blend type of U.S. Pat. No. 4,141,936, including those especially when the EVA and/or E-VA-VOH component possesses more than about 55 percent by weight of interpolymerized vinyl acetate, however, have a tendency towards instability to sunlight and retain impact strength for only a limited time upon exposure to ultraviolet rays.